As known in the art, an admixture is a material other than hydraulic cement, water, fiber reinforcement and aggregates that is used as an ingredient of concrete or mortar and is added to the batch immediately before or during its mixing. Admixtures are used to modify the properties of the concrete in such a way as to make it more suitable for a particular purpose or for economy. Thus, the major reasons for using admixtures are: (1) to achieve certain properties in concrete more effectively than by other means; (2) to maintain the quality of concrete through the successive stages of mixing, trans-porting, placing, and curing during adverse weather or traffic conditions; (3) to overcome certain emergencies during concreting operations; and (4) to reduce the cost of concrete construction. In some instances, the desired result can only be achieved by the use of an admixture. In addition, using an admixture allows the employment of less expensive construction methods or designs and thereby offsets the costs of the admixture.
For example, at the end of a day's delivery, concrete mixers may contain from 200 to 600 pounds of residual cement, sand or rock. When left in the mixer overnight, the residual concrete will settle and harden in the bottom of the mixer. While the residual materials can be washed out of the mixer with a large amount of water, disposal of the liquid may cause an environmental problem, particularly in large metropolitan areas. To avoid this problem, it is desirable to delay or retard the setting of residual concrete in a mixer so that it remains fluid and the residual material can still be used the next day. It is also desirable to be able to delay or retard the setting of concrete in a mobile mixer while the mixer is being transported to another location. For specific applications, it may also be desirable to retard or delay the setting of concrete for a specific length of time during breakdown or delay in traffic in populated areas. The addition of retarding admixture to the concrete is used to solve each of these problems. By varying the amounts of a retarding admixture used in a batch, the setting of the concrete can be delayed for a selected time period. The availability of a pre-weighed or pre-measured quantity of retarding admixture can increase the accuracy with which the setting time can be delayed.
Admixtures are commercially available as water-soluble solids or powders, requiring job mixing at the point of usage, or as ready-to-use liquids added at bulk blending station. The successful use of admixtures depends upon the accuracy with which they are prepared and batched. Batching means the weighing or volumetric measuring of the ingredients for a batch of either concrete or mortar and introducing them into the mixer. The amount of admixture added during batching must be carefully controlled. Inaccuracies in the amount of admixture added can significantly affect the properties and performance of the concrete being batched and even defeat the original purpose of including the admixture. The need for accuracy in measuring the amount of solid admixture to be added to a batch is particularly acute where only a relatively small amount of admixture is required for the job.
For powdered solid admixtures, it is particularly cumbersome to weigh the required amount of solid admixtures at the job because an additional scale or weighing apparatus must always be kept handy. At the job site, it is highly desirable to be able to add a known amount of solid admixture, preferably pre-measured in a bag, to the wet mixer and eliminating the requirement of weighing the admixture during batching. The use of pre-measured bags of concrete admixture not only minimizes human error in handling and pre-weighing the solid admixtures, it also facilitates the process of mixing. The biggest drawback of conventionally pre-measured and bagged admixtures is that opening and emptying the bags into the mixer creates a mess and results in wasting a certain amount of the admixture material which contributes to inaccuracies in batching.